Steam Generator

ABSTRACT

A steam generator which may be mobile or stationary, which includes a fire box the outlet of which is connected to a cyclone section; the outlet of the cyclone section opens into a water tube steam boiler, with the connection between the cyclone section and the water tube steam boiler being such that the cyclone section substantially isolates the water tube steam boiler from the radiant heat of a fire in the fire box; a structural shell contains the fire box, cyclone section and water tube steam boiler and is arranged to both provide a containment vessel if the contained components fail in use and also to bear any of the structural loads applied to the generator in use.

FIELD OF THE INVENTION

The present invention relates to a steam generator. The generator of the present invention is especially designed to be fuelled by a solid fuel, in particular by wood or a similar solid bio-mass fuel, e.g. bagasse, nut shells. However, the steam generator of the present invention could also be fired by solid fossil fuel such as coal or coke, or by a liquid or a gas fuel, including impure or contaminated liquid or gas fuels.

The steam generator of the present invention has been designed especially for use as part of a vehicle, (e.g. locomotive, tractor or truck) and will be described with particular reference to that application. However, the steam generator of the present invention could be used in any of a wide range of applications, and is not limited to vehicular use.

BACKGROUND OF THE INVENTION

The discussion of the prior art does not imply that the prior art discussed is part of the common general knowledge in the relevant fields.

In a steam powered vehicle, fuel is burnt in a firebox, the combustion heat is used to raise the temperature of water above 100 degrees centigrade in a boiler to produce steam. The steam is used to drive one or more pistons; known types of linkage can be used to convert the linear motion of the pistons to a rotary motion of the vehicle wheels. Alternatively, the steam can be used to drive one or more turbines, in known manner.

Hitherto, a majority of locomotives have been designed to burn fossil fuels in the form of coal, coke or oil. These fuels are highly calorific when they burn and tend to burn evenly and reliably with known thermal qualities. A further advantage is that coke and oil fuels tend to produce fewer sparks, and coal can be pre-cleaned to reduce the risk of sparking. Sparks are notorious for causing fires along railway tracks, and biomass fuels are considered to be a much higher sparking risk than fossil fuels. Sparks also represent a loss of fuel, reducing the efficiency of locomotive boilers by as much as 50% at high steaming rates.

One known technique for reducing the carryover of fuel particles (sparks) in combustion gases is to direct furnace gases into a cyclone, which can act either as a secondary furnace, to burn fuel particles and/or as a collector to agglomerate and collect both fuel and ash particles. Since it is the fuel particles which cause the sparking, the use of a cyclone in this way would also reduce the risk of sparking. An arrangement of this type is disclosed in U.S. Pat. 280 4854, which relates to a stationary furnace.

A further known drawback of biomass fuels is that they tend to produce less heat when burnt, thus requiring a much higher engine efficiency to be practical. However, burning fossil fuels is undesirable from an environmental perspective, and fossil fuels are becoming scarce and more expensive. It follows that, if the known drawbacks of biomass fuels could be overcome, they would be an attractive proposition as a fuel, given that biomass fuels when used to fuel conventional steam boilers are readily available, quickly replaceable, and cheap compared to fossil fuels.

Conventional steam locomotives have been predominantly made with boilers of the fire tube type.

As used herein, the term “fire tube boiler” means a boiler in which the hot combustion gases from the firebox are passed through tubes in the boiler; the tubes are surrounded by a mass of water contained in the boiler, and the heat in the tubes heats this mass of water into steam. Water also is heated by circulating water around the water space of a double-walled firebox; which is hereinafter referred to as a “wet” firebox. Typically, about 30% of the boiler’s steam output is generated from the wet firebox.

As used here, the term “water tube boiler” means a boiler in which the hot combustion gases from the firebox pass over the outer surface of one or more tubes contained in the boiler; water to be heated into steam is circulated through these tubes and the transfer of heat takes place convectively. Typically most designs of water tube boiler also transfer heat radiantly in a “wet” firebox, the firebox being constructed of tubes in which water to be heated into steam is circulated.

Fire tube boilers are expensive to construct compared to water tube boilers, and also are much more expensive to maintain. Also, because a fire tube boiler contains a large volume of water, considerable time is required to heat this water from cold up to a working pressure of steam at the start of the working period. Conversely, at the end of the working period, a large volume of heated water is left to cool in the boiler, which represents a significant waste of energy. A further drawback to this design is that the large diameter of the boiler limits the maximum pressure which can be generated inside the boiler to a comparatively low level (typically around 350 pounds per square inch) which is too low to achieve high engine efficiencies. The presence of what is effectively a large pressure vessel, (i.e. boiler) and the high temperature of the water it contains when in use, also presents a severe safety risk.

The above drawbacks notwithstanding, locomotive engineers have in general considered that the large volume of heated water provided by a fire tube boiler is in fact essential for handling the rapid changes in load which are characteristic of steam driven vehicles. For many years it has been received wisdom that the comparatively low volume water tube boilers simply will not perform satisfactorily in steam driven vehicles.

In a majority of locomotive designs, the pressure system components such as the heat exchanger portion of the locomotive (whether water tube or fire tube) form an essential part of the load bearing structure of the locomotive. This means that, in addition to the stresses on the heat exchanger caused by the pressure of the contained fluid and the expansion/contraction from heating the components, there are the structural stresses caused by the movement of the locomotive. This greatly increases the overall stresses, and can make a significant contribution to system failure.

OBJECT OF THE INVENTION

An object of the present invention is the design of a compact, lightweight solid fuel steam generator which can combust solid fuels, including biomass solid fuels, to generate steam safely and efficiently, whilst reducing harmful emissions.

A further object of the present invention is to achieve this within the general format and layout of conventional locomotive type boilers, so that the invention may serve as a direct replacement.

Another object of the present invention is the design of a solid fuel steam generator in which the boiler shell is functionally independent of the pressure system components, so that the boiler shell can be a structural element of the generator without imposing additional stresses on the pressure system components.

SUMMARY OF THE INVENTION

The present invention provides a steam generator which includes:

-   a fire box in which fuel may be burnt; -   a cyclone section connected to the firebox such that the inlet into     the cyclone section communicates with the outlet from the firebox; -   a water tube steam boiler connected to the outlet of the cyclone     section such that gases from the cyclone section pass into the     boiler; -   the connection between the cyclone section and the water tube steam     boiler being such that the cyclone section substantially isolates     the water tube steam boiler from the radiant heat of the fire in the     firebox in use; -   a structural shell adapted to contain the firebox, cyclone section     and water tube steam boiler, said shell being constructed to provide     a containment vessel if any of the firebox, cyclone section and     water tube boiler fail in use; -   said structural shell further being adapted to bear any of the     structural loads applied to the steam generator in use, such that     the firebox, cyclone section and water tube steam boiler do not act     in use as structural members of the steam generator.

Preferably, the cyclone section is a single cyclone. However, the cyclone section may consist of more than one cyclone, with the cyclones arranged either in series or in parallel. The water tube steam boiler may be a mono tube boiler or a multi-tube boiler, and preferably is a forced circulation boiler.

The flow of combustion gases through the boiler may be radial or axial or some combination of the two, but preferably is an axial flow (i.e. substantially parallel to the length of the boiler) or an axial flow in combination with a plurality of radial flows along the length of the boiler.

Preferably, the steam generator further includes a steam separator connected to a steam outlet from the boiler.

Preferably also, the water drained from the steam separator is recirculated back into the boiler. The water drained from the steam separator may be fed to a feedwater heater.

Preferably also, the steam generator further includes a steam superheater.

Preferably, the steam generator also includes a smoke box formed by one end of said structural shell, the smoke box arranged such that gases which have passed through the boiler pass into the smoke box, the smoke box being provided with a flue for venting said gases, and with a draught generating arrangement to create a draught through the flue.

Preferably, the inlet section for the or each cyclone of the cyclone section includes a solid central portion surrounded by a plurality of peripheral vanes, such that gases passing from the firebox outlet to the cyclone section in use must pass through said vanes; each of said vanes being angled at an acute angle to the plane of said solid central portion. The angle of each vane may be adjustable. Preferably, each of said vanes is angled to the plane of said solid central section at an angle in the range 5° -80°, most preferably in the range 20° - 30°. The purpose of said vanes being to generate cyclonic flow within the cyclone section.

Preferably, the structural shell takes the form of a conventional firetube “Locomotive Boiler” as illustrated in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a preferred embodiment of the present invention is described in detail, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic vertical section through a solid fuel steam generator in accordance with the present invention;

FIG. 2 is a section on line 2.2 of FIG. 1 ;

FIG. 3 is a section on line 3.3 of FIG. 1 ;

FIG. 4 is a simplified version of FIG. 1 , including a diagram of a control system; and

FIG. 5 is a schematic side view of a multi-tube boiler.

BEST METHOD FOR CARRYING OUT THE INVENTION

Referring to the drawings, a solid fuel steam generator 10 consists of three main sections: a firebox 11, a cyclone section in the form of a single cyclone 12 and a boiler 13. These three sections are depicted as being horizontally arranged, but this is not essential: they can be vertically arranged, i.e. with the cyclone vertically above the firebox and the boiler vertically above the cyclone. The firebox 11, cyclone section 12 and boiler 13, plus ancillary equipment as hereinafter described, are contained inside a structural shell 10 a that contains and protects these components, and serves as the principal structural member of the steam generator, to minimise the structural stresses on the components contained in the shell. The firebox and/or the cyclone may be provided with cooling jackets (not shown) through which water, air or other cooling medium is circulated; the resulting heated medium may be used to preheat combustion air or boiler feedwater.

However, the heated medium is used primarily for cooling the firebox and/or the cyclone, rather than for the generation of steam. This type of arrangement is known as a “dry” firebox.

Use of a ‘dry’ firebox means that substantially all of the heat produced in the firebox is available for fuel combustion:- almost no heat is dissipated to the firebox walls, to heat water in the walls, as in a ‘wet’ firebox. This means that combustion of the fuel is held at a high temperature until the combustion reaction is complete, resulting in a marked reduction of pollutants.

In a ‘wet’ firebox, the firebox walls, being water-filled, remain cool, and thus tends to quench the combustion gases where they contact the walls. This causes volatile organic compounds and soot to be entrained in the firebox gases.

The use of a dry firebox is possible because of the adoption of a low-volume forced circulation heat exchanger (as described below), which provides efficient transfer of heat convectively from the combustion gases to the boiler water.

The firebox 11 is an enclosure made of heat resistant material, (e.g. steel, stainless steel) and may be coated inside and/or outside with a high-temperature resistant refractory coating. The firebox is fitted with a grate 14 on which fuel to be burnt rests. An ash receiver 15 and an ash discharge door 16 are located below the grate, in known manner (not shown in FIG. 4 ). The firebox 11 is roughly rectangular in cross-section, but with the lower wall 11 a adjacent the firebox outlet 17 inclined at an obtuse angle to the plane of the grate 14, to provide a smooth transition for gases from the firebox into the adjacent cyclone 12, and to avoid excessive heating at the adjacent part of the cyclone 12.

Secondary air inlets 11 b may be formed in the lower wall 11 a, and this also assists in reducing any excessive heating at the adjacent part of the cyclone 12.

The ash receiver 15 tapers in cross-section from the grate 14 down to the ash discharge door 16.

The firebox 11 is provided with a door 18 in the wall of the firebox opposite the cyclone 12; in use, fuel is fed in through the door 18 and falls on the grate 14. The door 18 is pivoted at a pivot 19 and, when opened, pivots inwards into the firebox. When the door 18 is partially open, it directs a flow of secondary combustion air into the firebox.

The door 18 may be a single door or may be multiple doors. Depending upon the type of fuel feed system to be used, the door may be located where shown in FIG. 1 or on the top or sides of the firebox 11. Another possibility is to supply fuel as an underfeed with a mechanical stoker.

Airflow into the steam generator is marked by double headed arrows: air is admitted into the firebox when the door 18 is open, and the door 18 also may be formed with vents through it or around it. Air also is admitted through a plurality of inlets 20 around the underside of the grate 14 (with or without damper control (not shown)). The design shown in the drawings does not provide for the preheating of the combustion air, but it is envisaged that this may be advantageous, and this feature could be added, e.g. using engine exhaust steam or other waste heat source, or by an air jacket around the firebox.

The firebox outlet 17 forms the inlet section for the cyclone 12. Referring in particular to FIG. 2 , the outlet 17 is circular in cross-section and has a solid centre 22 surrounded by a series of peripheral angled vanes 23. The vanes may be cooled internally (not shown).

Each vane 23 is angled at an acute angle to the plane of the centre 22; angling the vanes in this way imparts a swirl to the combustion gases (shown by single-headed arrows in FIG. 1 ) and entrained particles leaving the firebox as they enter the cyclone 12, to form a vortex. Each vane 23 may be angled to the plane of the centre 22 at an angle in the range 5 - 80 degrees, but an angle in the range 20 - 30 degrees is preferred. The angle may be adjustable. The vanes 23 may be curved and/or contoured to improve aerodynamic performance, in known manner.

The cyclone 12 is circular in cross-section, with an inverted frusto conical portion 24 at the cyclone outlet. In the cyclone, the swirled combustion gases circulate around the cyclone, with the heavier entrained particles moving out towards the outer wall of the cyclone. The cyclone may be coated on the inside and/or outside with a high-temperature-resistant refractory material, and is formed with an opening (not shown) at its lowest point to allow the removal of any accumulated ash.

An alternative cyclone outlet design is shown in broken lines in FIG. 1 : the inverted frusto conical portion 24 is replaced by a straight sided outlet 24 a which is rectangular in plan.

Essentially, the cyclone provides a delay between the combustion gases leaving the firebox and entering the boiler 13, and this delay allows for full combustion of the gases and any entrained fuel particles. Additional air may be admitted into the cyclone to ensure that sufficient oxygen is present for full combustion. This air may be admitted tangentially, to aid the spiral flow of air in the cyclone. Achieving full combustion improves the overall efficiency of the system, and also means that the cyclone acts as a spark arrestor, in that all or nearly all sparking material completes its combustion in the cyclone and cannot be carried through the boiler to enter the exhaust system; this effectively eliminates the fire risk from sparks. In addition, because most entrained particles are completely combusted in the cyclone or are deposited in the cyclone, this eliminates the problem of solid particles being carried into the boiler 13 and clogging the gas passages between the turns of the boiler tube, and/or causing mechanical erosion of the heating surfaces.

In addition, it would be possible to admit steam tangentially into the cyclone, if it proves necessary (under some operating conditions) to maintain or increase the velocity of the gases in the cyclone.

An additional advantage of the positioning of the cyclone is that the cyclone separates the firebox from the boiler, and thus separates the boiler tube or tubes from the radiant heat from the firebox. This helps to prevent localised overheating of the boiler tube or tubes. Also, this arrangement makes the steam generator more controllable because the supply of heat to the boiler is governed purely by the flow of gas through the boiler, and not by the radiated heat from the fire in the firebox, which is not directly proportionate to the load on the boiler. Thus, the separation of the fire box from the boiler by the cyclone section overcomes the main objection to a low volume water-tube boiler, that of variable load.

The swirling action of the gases in the cyclone results in a substantially uniform temperature of the gases exiting the cyclone. This improves the conditions under which the boiler operates, for example by prolonging the life of the boiler by subjecting it to a more even heat flux. This is especially valuable when a multi-tube boiler is used: in a multi-tube boiler, an uneven heat flux can cause instability of flow in some tubes of the boiler, and overheating as a result.

The boiler 13 is depicted in FIGS. 1 - 4 as a mono tube boiler with a single tube 25 wound in a multilayer spiral around a central cylinder 26. However, the boiler could be a multi-tube boiler as described hereinafter with reference to FIG. 5 . In either case, the boiler is a “water tube” type i.e. water is circulated through the tube or tubes, to be heated by the hot gases exiting the cyclone and passing through the gaps between the coiled mono tube or between the multi tubes.

It should be noted that steam locomotives fuelled by a biomass solid fuel have hitherto predominantly been of the “fire tube” type (i.e. where the combustion gases pass through tubes surrounded by the water to be heated) because, along with the reasons described above, combustion of biomass fuels tends to produce a relatively large proportion of entrained solid particles, and these notoriously tend to clog the air spaces between the turns of the mono tube or adjacent tubes, leading to a reduction in steaming capacity.

For the boiler tube, a spiral wound mono tube is the preferred arrangement because this construction provides a high efficiency of heat exchange and is relatively inexpensive to manufacture. It is envisaged that the tube will be made of steel or stainless steel. The diameter of tube used depends upon the boiler diameter; for example, for a boiler diameter of 240 mm, an outside tube diameter of 12.7 mm has been found suitable. One possible construction method for the mono tube boiler is to form a series of spiral-wound tube units, and connect these units end-to-end to form a single continuous tube. Hitherto, mono tube boilers predominantly have been used with liquid or gas fuel, because these fuels permit easy control of the firing rate and thus regulation of the steam temperature. Regulating temperature by controlling the firing rate is not a practical proposition for a solid fuel fired firebox, where it is preferable to be able to use a constant firing rate and regulate the steam temperature by alternative control means, as described below.

Water to be heated into steam (“feedwater”) is fed into one end of the mono tube through inlet 27 and passes around the full length of the spiral, being heated as it does so by the hot combustion gases (shown by a single headed arrows) passing through the interstices of the spiral wound layers of the mono tube. Steam leaves the boiler through outlet pipe 28 and then travels through a pipe 29 down the centre of cylinder 26 to an outlet 30 at the end of the boiler 13.

The outlet 30 could be connected directly to the equipment requiring the steam, (e.g. engine pistons or turbines) but preferably is directed first to a steam separator and then to a superheater, as described below with reference to FIG. 4 .

In the embodiment illustrated in the drawings, the flow of the hot combustion gases through the boiler is substantially axial, i.e. along the longitudinal axis of the boiler. It is believed that an axial gas flow through a relatively long and small diameter boiler will provide the highest heat transfer coefficient and thus the highest efficiency. However, for other shapes of boiler and/or alternative boiler tube arrangements, other gas flow patterns may be preferred. For example, the hot combustion gases could be passed through a perforated tube in the centre of the boiler and allowed to percolate out through the perforations in the tube and so through the gaps between the tube windings.

Once the combustion gases have passed right through the boiler, they enter a smoke box 31 and pass out of a flue 32. Steam exhausted from the pistons may be fed back into the smoke box 31 to create a draught up the flue 32, in known manner.

FIG. 5 shows a schematic view of a multi-tube boiler 40. In this construction, two identical tubes, 41, 42, both spirally wound, are fed with water to be heated to steam via a supply line 43 which supplies two branches 44, 45, connected respectively to the tubes 41, 42. Each branch 44, 45 is provided with a reduced diameter orifice 46, 47.

In a multi-tube boiler, it is important to ensure that all tubes are supplied with a substantially equal quantity of water to prevent either a dry tube condition, or an excessively flooded tube condition. This can be achieved either by providing the supply line with a reduced diameter orifice as described above or by supplying each line by means of a separate pump and a separate temperature control system.

Using a reduced diameter orifice is the simpler solution to the problem: because the orifice has a reduced diameter, it produces a significant pressure drop in water fed through the tube, so that small pressure changes within the tube do not affect the delivery of water to those tubes.

The multi-tube boiler may have more than two tubes. Control of the multi-tube boiler is achieved in the same manner as for a mono tube boiler, as described below.

For safe and efficient operation of any steam generator, it is necessary to control the supply of water to the boiler (feed water), the steam temperature, the steam pressure, and the firing rate, (i.e. the rate at which fuel is burned in the firebox). A range of known control systems could be used to control the above described steam generator, and the systems hereinafter described are simply one possible way of achieving the necessary control. The control systems are described below with reference to a mono tube boiler, but are equally applicable to a multi-tube boiler.

Referring in particular to FIG. 4 , feed water control is by means of a steam operated pump 50 which feeds water from a water tank 65 or other supply through a check valve 52 to the inlet 27 of the boiler tube 25. The feed-water optionally may be pre-heated in a feed-water heater (not shown) between the tank 65 and the inlet 27. The pump 50 preferably is a steam driven direct acting piston type, and the rate at which feed water is supplied to the boiler tube is controlled by starting and stopping the supply of steam to the pump 50, as hereinafter described. The check valve 52 prevents water in the boiler tube 25 flowing out of the boiler and back towards the pump 50.

Once the feed water enters the boiler tube 25, it passes around the coils of the tube, being heated as it travels through by the hot combustion gases, as described above. By the time the boiler tube outlet 30 is reached, the feed water is predominantly steam, although there will be some water entrained with the steam. At this point, the steam passes a temperature probe 56, which is connected to a temperature controller 57. The temperature controller 57 is arranged to turn on the steam supply to the water pump 50 when the temperature measured by the temperature probe 56 exceeds a predetermined temperature (typically slightly above the saturated steam temperature at the maximum boiler pressure). The pump 50 then is operated until the water level in the tube 25 is pushed almost to the outlet 30; this cools the temperature probe 56 and the temperature controller 57 then shuts off the pump 50.

It is advantageous to set the control system for the pump 50 so that a slight excess of water is provided to the boiler tube 25, because the flow of water through the tube 25 carries away impurities and also provides a maximum heat transfer coefficient; this provides a steady steam pressure from the boiler.

Steam coming from the boiler tube outlet 30 may of course be used directly for any application requiring steam. However, for use in a steam locomotive, it is preferable to superheat the steam. Firstly, the steam/water leaving the outlet 30 is passed into the steam/water separator 55 to “dry” the steam, i.e. to remove the entrained water. Typically, the separator 55 is a cyclonic separator of known type. The separated dry, clean steam leaves the separator 55 through steam line 60, and the separated hot water accumulates in the base of the separator 55.

The steam line 60 is connected to a superheater element 61, which may be any of a number of known types of superheater. For example, the superheater may be in the form of a water tube 62 (FIG. 1 ) which is arranged as a coil between the cyclone exit and the boiler tube. The hot combustion gases will pass over the walls of the superheater tube immediately after leaving the cyclone, superheating the steam. The temperature of the steam entering and/or leaving the superheater may be measured using temperature probes (not shown) in known manner. To regulate the temperature of the steam leaving the superheater, condensed water may be injected in controlled quantities. Condensed water is used because it is distilled and thus clean and free of impurities.

The steam leaving the superheater is supplied to the engine pistons or turbines (shown in FIG. 4 as box 80).

The usual arrangement for draining water from the separator is to provide a float valve in the separator, which automatically opens to drain water when a certain amount of water has accumulated. However, the system has the disadvantage that it occupies space in the separator.

In the present invention, water is drained from the separator 55 through a manual valve 63 which is on the exit line 55 a to the feedwater tank 65. Opening the valve 63 allows water to drain through a reduced diameter orifice 64; the presence of the reduced diameter orifice 64 maintains the pressure in the system. The water drained through line 55 a is shown as draining to the feedwater tank 65, but may in fact drain to a feedwater heater, if present, or to a hot-well (not shown).

The diameter of the orifice 64 is set so that the drainage from the feedwater heater is slightly greater than the quantity of excess feed water supplied by the pump 50 - this ensures that all water is drained from the separator.

The valve 63 can be closed to conserve boiler pressure when the boiler is not being fired for a period.

It will be appreciated that the above described drainage system is extremely simple, and has the additional advantage of constantly draining impurities from the separator.

Further, the above system does not require any equipment actually in the separator itself, and hence the separator 55 can be smaller than would otherwise be possible. Alternatively, various steam traps of known types may be employed to perform a similar function.

The water pump 50 is controlled by a steam solenoid valve 67, which controls the supply of steam to the pump 50; the steam line 68 supplying the pump 50 is depicted as connected to the steam line 60 from the steam separator 55, but may in fact be connected to any convenient part of the steam supply.

As an alternative to the above arrangement, it is also possible to operate the pump 50 continuously, and control the feedwater supply by placing a by-pass valve in the water supply line, and turning this valve off and on as required. If the system is adopted, then the remainder of the system works as described above, in that turning on the feedwater supply increases the pressure in the system.

Turning now to the control of the steam pressure and the control of the firing rate, these two factors are linked, because the steam pressure is in fact controlled by the firing rate, i.e. the rate at which fuel is burnt in the firebox. The firing rate is controlled by the draught through the system.

It is possible to force combustion gases through the system by using a pressurised air supply into the firebox 11; if this system is adopted, particulate fuel generally is blown in with the pressurised air supply. However, such a system is relatively complex and is not preferred for the present application, although it would be possible to adopt it.

Preferably, air and combustion gases are drawn through the system by the reduced pressure developed in the smoke box 31 by the draught system, during operation of the steam generator.

Draught is generated by providing, in known manner, an exhaust nozzle 70 located in the smoke box 31 adjacent the lower end 71 of the flue 32. The nozzle 70 is supplied with steam from the engine exhaust, which is directed to pass straight up the flue 32 and hence draw the combustion gases from the firebox 31 and up the flue 32.

The amount of draught provided by the nozzle 70 is provided by a system that varies the back pressure on the engine by adjusting the area of the exhaust nozzle 70 depending upon the boiler pressure, so that as boiler pressure increases, the nozzle area increases and reduces engine back pressure, therefore reducing the draught slightly. For the most part the draught generation with this system is proportional to the steam consumption; the adjustment of back pressure ensures maximum efficiency without wasting energy as excess draught.

The flow of gas through the flue 32 may be boosted when necessary by providing a perforated coil 72 which surrounds the nozzle 70, and which is arranged to be provided with steam (e.g. from the steam separator 55), to provide a rapid and controllable boost to the rate of flow of the gases through the flue 32. Steam is supplied to the coil 72 only when required, and can be manually or automatically controlled.

A rather more effective arrangement is to substitute for the perforated coil 72 a ring of high pressure steam expansion nozzles which receive steam at boiler pressure through a manual or an automatic valve.

The rapid and accurate control of the steam generator is assisted by the fact that the boiler tube is isolated from the radiant heat of the fire in the firebox 11, and thus the rate of steam generation from the boiler depends primarily upon the convective heat transfer from the combustion gases to the water/steam in the boiler tube. Thus, if the draught, (i.e. the flow of gases through the flue 32) is reduced by reducing the amount of steam supplied to the nozzle 70 or coil/nozzles 72, this immediately results in a reduction in steam generation; this would not be possible if the boiler tube were exposed to the radiant heat of the firebox.

The above described system is capable of operating over a very wide range of boiler pressures, up to several thousand pounds per square inch absolute (psia). This means that the saturated steam temperatures have an equally wide range - for example, if the boiler were operating at a very low pressure, the saturated steam outlet temperature could be as low as 100° C., but more typically, the boiler would be operating at between 300 psi and 1000 psi, with a saturated steam temperature of between 200° C. and 285° C. Commonly, the superheated steam temperature for locomotive applications is about 350-450 degrees centigrade.

Consolidating the various systems previously described is a structural shell 10 a that takes the form of the shell of a conventional firetube locomotive boiler. This shell performs several key functions:

-   it packages the firebox, cyclone section, water-tube boiler, steam     separator and smoke box, and associated equipment in a compact     manner with a small frontal area, reducing air resistance as applied     to steam driven vehicles; -   it is also directly compatible with existing designs of steam driven     vehicles, permitting its use as a drop-in replacement on, for     example, railway, road and agricultural locomotives. -   steam driven vehicles typically employ the steam generator as a     primary or auxiliary structural member. Historically, when     water-tube boilers were adapted for locomotive use the pressure     system components also served as structural members. It was found     that cyclic loads generated under the dynamic conditions of     locomotive use soon gave trouble with leaking and cracked pipework.     The lack of conventional water-tube boiler rigidity also compromised     the structure of the locomotive as a whole. The structural shell     provided by the present invention overcomes this difficulty by     relieving the entire pressure system of any loads other than that of     the mass of the pressure system components themselves. -   a further function of the structural shell is that of safety. The     structural shell and the pressure system are designed in such a way     that in the event of a catastrophic pressure system failure, the     maximum pressure exerted on the shell will not exceed an entirely     safe pressure of 0.05 MPa. The shell thus functions as a containment     vessel, safely containing and dissipating the energy released upon a     pressure system failure. This is achieved by taking the internal     volume of the shell and calculating from it the maximum volume of     steam and water that if released from the pressure system all at     once, will not exceed the aforementioned safe pressure within the     shell of the locomotive steam generator. -   in addition, the structural shell protects the components contained     inside it and seals against rain/dust or accumulations of other     airborne particulate material which in combination would otherwise     cause severe corrosion of pressure system components. Importantly,     the structural shell prevents ambient air from leaking into the     firebox, cyclone section or boiler which in sufficient quantities     would cool the combustion gas, reducing the efficiency of the steam     generator. 

1. A steam generator which includes: a firebox in which fuel is burnt to produce heat and gases that are expelled through a firebox outlet; a cyclone section connected to the firebox wherein the cyclone section has a cyclone section inlet and a cyclone section outlet, and wherein the firebox outlet communicates with the cyclone section inlet so that the gases from the firebox outlet pass into the cyclone section; a water tube steam boiler connected to the cyclone section outlet of the cyclone section such that the gases from the cyclone section pass into the water tube steam boiler wherein the cyclone section substantially isolates the water tube steam boiler from the radiant heat of the fire in the firebox in use; a structural shell adapted to contain the firebox, the cyclone section and the water tube steam boiler, said shell being constructed to provide a containment vessel for the firebox, the cyclone section and the water tube boiler in use; wherein said structural shell further bears structural loads applied to the steam generator in use, such that the firebox, the cyclone section and the water tube steam boiler are isolated from the structural loads applied to the steam generator in use.
 2. The generator as claimed in claim 1, wherein the cyclone section consists of a single cyclone.
 3. The generator as claimed in claim 1, wherein the cyclone section consists of at least two cyclones, with cyclones arranged either in series or in parallel.
 4. The generator as claimed in claim 1, wherein the water tube steam boiler is a mono tube boiler.
 5. The generator as claimed in claim 1, wherein the water tube steam boiler is a multi-tube boiler.
 6. The generator as claimed in claim 1, wherein the water tube steam boiler is a forced circulation boiler.
 7. The generator as claimed in claim 1, wherein the water tube steam boiler is designed to permit the gases to flow therethrough in use in an axial flow direction, substantially parallel to the length of said boiler.
 8. The generator as claimed in claim 1, wherein the water tube steam boiler is designed to permit the gases to flow therethrough in use in an axial direction, substantially parallel to the length of the boiler and also in a plurality of radial directions, substantially perpendicular to the axial direction, at a plurality of positions along the length of the boiler.
 9. The generator as claimed in claim 1, wherein the water tube steam boiler includes at least one spirally wound boiler tube.
 10. The steam generator as claimed in claim 1, wherein the firebox is a dry firebox.
 11. The steam generator as claimed in claim 1, wherein the firebox, the cyclone section, and the water tube steam boiler are positioned adjacent each other along a horizontal axis.
 12. The steam generator as claimed in claim 1, wherein the firebox, the cyclone section and the water tube steam boiler are positioned adjacent each other along a vertical axis.
 13. The steam generator as claimed in claim 1, further including a steam separator connected to a steam outlet from said water tube steam boiler.
 14. The steam generator as claimed in claim 13, wherein water is drained from said steam separator and is recirculated to the water tube steam boiler.
 15. The steam generator as claimed in claim 13, further including a steam superheater which is connected to the steam separator so as to receive steam from the steam separator in use.
 16. The steam generator as claimed in claim 1, further including a smoke box formed by f said structural shell, the smoke box being arranged such that in use, the gases which have passed through the boiler pass into the smoke box, the smoke box being provided with a flue for venting said gases, and with a draft generating arrangement to create a draft through the flue in use.
 17. The steam generator as claimed in claim 1, wherein the the cyclone section inlet for each cyclone section includes a solid central portion surrounded by a plurality of peripheral vanes, such that the gases passing from the firebox outlet to the cyclone section in use pass through said vanes, each of said vanes being angled at an acute angle to the solid central portion.
 18. The steam generator as claimed in claim 17, wherein each of said vanes is angled to the solid central portion at an angle in the range 5° - 80°.
 19. The steam generator as claimed in claim 18, wherein each of said vanes is angled to the solid central portion at an angle in the range 20° - 30°.
 20. The steam generator as claimed in claim 17, wherein the angle of each of said vanes is adjustable.
 21. (canceled) 